Do you’ve got a link? Are you using mod_rewrite / .htaccess in any way? Have you tried to enable PHP errors?
Check these:
1 - Regular link to entry with index.php, template-group, template, entry
2 - Broken link to a non-entry with index.php, template-group, template, gibberish (not an entry)
3 - Broken link to non-entry with index.php, wrong template-group name, wrong template name, gibberish entry name (not an entry)
Not using mod_rewrite in .htaccess to remove index.php. Not seeing any PHP errors. With debugging on the string says 404 page is showing but it is not (see image enclosed). I’m using a number of path variables—does that make a difference?
#1 returns the entry appropriately and as expected.
#2 returns to the most recent entry.
#3 returns a blank white page (no code).
Whatever is happening the 404/error template does not display. I’ve verified this on a couple of other EE 1.6.9 installs. I suspect it is related to the path variables.